The invention relates to a method for the production of a ceramic tile, glazed or enamelled, and tile obtained by this method.
The first method requires the formation of the raw tile, which is successively exsiccated and fired. In this way, a manufactured article known as a biscuit is obtained.
On the biscuit a layer of compound, better known as frit, is applied. Suitable additives or pigments have been added to the frit. All of these materials undergo, successively, a further firing which has the purpose of inducing structural modifications in the frit, this depending, obviously, on the nature of the substances making up the frit. As a result there is a fusion of these substances, or else a partial fusion of these substances according to a well known process of liquid phase sintering. The final consequence is the formation of a vitreous layer on the upper facing of the tile.
In conclusion a double firing is had, firstly of the raw tile (or support) and then of the frit, from which the name, "double firing," of the above method is taken.
The rest of the method requires firing of the tile and the frit in a single operative phase, from which comes the well known name of this method of "single-firing".
During firing of the tile, great quantities of gas originate from the tile (example: carbon dioxide), which gas enters the surrounding environment (which consists of the oven itself). The gas comes out of the surface of the tile, and, in particular, through the layer of the superimposed frit. Passage of these gases through the frit is possible as long as the frit is permeable, that is to say, before the softening of the frit.
The glaze, or enamel, obtained from firing of the frit is usually impermeable.
It is known that the frit for a colorless glazed tiling is obtained by thorough mixing, and by grinding together some acid substances (silicon, boron, phosphorous, etc.) with some basic substances (compounds of sodium, of potassium, of calcium, of lead etc.).
These substances are brought to a melting point in a melting pot, then poured out and cooled all at once, usually by immersion in water.
By means of the above-mentioned cooling, lumps of frit are obtained. From these lumps, by means of grinding, frit is obtained in the form of powder or granules.
In a first technique, the powdered frit, in a watery emulsion, with possible additives and thickening agents, is applied on the tile to glaze, or to enamel, in various ways. For example, the frit is sprayed on, brushed or sprinkled, or is applied through systems of serigraphy etc.
In a second case, the granules of frit, with eventual additives and pigments, are applied on the tiles dry with the help of suitable glues. This is carried out both by "single-firing" and "double-firing". In the case of the "single-firing" a suitable pressing of the frit can be anticipated.
The present invention fits into a special technical sector regarding the glazing, or enamelling, of ceramic tiling by use of frit in a granular form.
The grinding of the lumps of frit, by means of which the above-mentioned granules are obtained, also involves the forming of dust. A suitable plant is thus made necessary for recuperating and recycling of the dust in order to avoid waste as well as intolerable levels of pollution.
This last disadvantage, because a percentage of very fine dust always remains suspended in the air, means in the long run, for the operators of the equipment, the onset of illnesses related to tile production.
Also, it is to be pointed out that the equipment used for the grinding is subject to wear and tear, which is accentuated by the hardness of the lumps of frit. Thus the equipment requires continuous maintenance as well as replacement of the equipment itself.
The aforesaid granules are different in their forms and dimension, thus making it necessary to make a selection, by means of sieving, for the purpose of selecting the granules presenting dimensions-included between two values, minimum and maximum (example: observing a predetermined ration between these two values).
The granules even after sorting are of an extremely irregular form, each granule is different from the other, which means a great many disadvantages, listed as follows:
In fact, the above-mentioned sorting does not exclude the presence of needle-shaped granules. Therefore, the possibility exists that some of these needle-=shaped granules jut out from the border of the tile onto which a layer of frit has been applied.
With movement of the tile, a portion of a needle-shaped granule, jutting out from the border of this tile, can be intercepted, causing rotation of the above-mentioned granule (with its probable falling off) in respect to a vertical axis. This causes the removal of a portio of the frit in the area "swept" by the granule as a consequence of the aforesaid rotation, and as a result an imperfect tile, namely a discard, may be produced.
The above disadvantage manifests itself both with the single-firing and double-firing method.
In some cases of single-firing, application on the tile of a layer of a mixture of frit, china clay, alumina, zirconium hydroxide (known as "engobbio") prior to the application of the final frit, is foreseen. The function of the layer is to create a chemical insulating layer between tile and glaze. Even in this case the disadvantage mentioned above can manifest itself due to the present of "needle-shaped" granules.
With the above-mentioned granules it is practically impossible to eliminate dust. In fact, a percentage of this dust, even if small, remains imprisoned in between the granules and tends to spread itself inside the kiln dirtying the kiln itself as well as the equipment (example: operative lines) contained in the kiln. As the experts in the art know very well; this dust necessitates frequent stoppage of this equipment for necessary cleaning operations, with consequent aggravation of maintenance costs.
Additionally, the dust deposits itself on the vault of the kiln and tends, due to the heat, to melt, dripping down onto the underlying tiles, as a consequence, causing discards.
Finally the presence of this dust means, even for the operators situated in the operative zones where the frit is utilized, the danger of getting work-related illnesses.
In the "single-firing" techniques, as already mentioned, during the firing of the tile, gas escapes from the tile, which gas is given out to the surrounding environment through the layer of frit. The permeability of this frit layer is not uniform because the granules are different one from another in form and size.
As the frit, when it is being glazed, becomes impermeable, for the reason mentioned above, the passage between the phase of permeability and that of impermeability comes about in a differentiated way from area to area. For example, the "needleshaped" granules, the dust, the small granules, becomes welded together and/or melt in the time before the melting of the remaining granules. This welding can cause occlusion of differentiated quantities of gas bubbles sin the coating of the tile, or else cause occlusion of bubbles in some areas and to complete degasification of the remaining area. In both cases, the coating of the tile has a relatively spongy quality which differs from area to area. All these disadvantages are well known to the experts of the art.
Differentiated melting of the granules can cause, when use is made of different frits, the formation of solid phases (so-called "unmelted compounds") that float on the frit. Once glazing has been completed, these solid phases can be distinguished as specks, which from an aesthetic point of view degrade the tile.
The varying granulometry of the frit is certainly not optimum with regard to application of this frit on the fired tile ("double-firing" or on the raw tile ("single-firing"). This results from the fact that the layer of frit which coincides with the underlaying facing of the tile is extremely irregular from area to area.
Levelling out of the layer of frit, in order to make the thickness of the layer itself constant, cannot be considered, as the jagged edges of the granules would cause them to be swept away by the equipment used for this levelling purpose with consequent forming of an undesired and unacceptable lining.
In this way, being unable to carry out levelling of this type, the thickness of the layer of frit is not uniform, with all the disadvantages arising from this, the substances used to carry out colorless glazing have already been discussed previously. In order to carry out colored glazing it is necessary, for example, to add (if they are not already part of the composition of the frit) metallic oxide colorings, for example, iron, oxide, cobalt, manganese, nickel oxide, etc. For the opalizing enamels, the addition is required, for example, of substances like tin bioxide, zirconium, fluorite, sodium fluoaluminate, and bone phosphate. In order to obtain special effects it is necessary to add particular additives having a base of boron, titanium, etc.
According to a known technique, the pigments (that is, the aforesaid oxides) and the additives are made up in a powdered form and applied, in the form of a thin veil, onto the granules of frit to which they are cemented, with a hot or cold process, by means of suitable glues.
Application of the undermentioned veil (pigments and additives) is influenced in a negative way by the interstices and jutting spikes of the layer of frit, as well as by the dust imprisoned in these interstices. Therefore, the thickness of the veil is not at all even, which is certainly not optimal for the results intended to be achieved.
The technique of carrying out decorations on a tile by means of granules on the layer of frit has been noted. The aesthetic effect, which is obtained once the glazing has been completed, depends on the form of a granule. A "needleshaped" granule and a granule of a different form, always of an irregular shape, give rise, respectively, to different aesthetic effects. In fact the "needleshaped" granules make the outline of the designs appear smudged.
The screens used for some types of decorations (example: serigraphic procedures) are subject to a great deal of wear, because of the abrasive action on these screens by the jagged points of the granules. This high wear, considering the cost of the screens themselves, which is not low, affects in a negative way the unit cost of the tile.
The holders, which are used for the distribution of the granules of frit, must be subject to constant maintenance as a consequence of their wearing out. This wear is accentuated by the jagged points present in the above-mentioned granules.
According to a further known technique, the aforesaid pigments and additives, in a powdered form, are mixed with the frit in the form of powder. The above-mentioned mixture of powders is then successively fixed, through suitable glues, making granular elements in an extremely irregular form, as well as having craters. As a result, the disadvantages mentioned above are noticeable.
Besides, the above-mentioned elements, during their manipulation, tend to free dust as a consequence of the inevitable breaking up of some of these elements; this affects, in fact, mixtures of powders which are not entirely agglomerated.